The optical properties of nanoapertures in an optically thick metallic film have been intensely researched in the past several years due to their fundamental importance in near-field optics, and their practical significance to photonic devices and applications, including filters, near-field probes, and optical data storage. For the latest developments, see for example the focus issue in Optics Express 12, 3618 (2004). It is well-known that the transmission characteristics are strongly influenced by the presence or absence of propagating modes inside the apertures. J. A. Porto, F. J. Garcia-Vidal, and J. B. Pendry, Physical Review Letters 83, 2845 (1999); F. I. Baida and D. Van Labeke, Physical Review B 67 (2003). In metallic nanoslits, enhanced transmission has been attributed to propagating transverse magnetic (TM) modes inside the slits. J. A. Porto, F. J. Garcia-Vidal, and J. B. Pendry, Physical Review Letters 83, 2845 (1999); Y. Takakura, Physical Review Letters 86, 5601 (2001); E. Popov, M. Neviere, S. Enoch, and R. Reinisch, Physical Review B 62, 16100 (2000); S. Astilean, P. Lalanne, and M. Palamaru, Optics Communications 175, 265 (2000); P. Lalanne, J. P. Hogonin, S. Astilean, M. Palamarn, and K. D. Moller, Journal of Optics A: Pure Applied Optics 2, 48 (2000); Q. Cao and P. Lalanne, Physical Review Letters 88 (2002). For cylindrical holes, such as those featured in Ebbesen et al.'s original experiments, T. W. Ebbesen, H. J. Lezec, H. F. Ghaemi, T. Thio, and P. A. Wolff, Nature 391, 667 (1998), the spectral features have been shown to be largely independent of the material used for the vertical walls of the hole. D. E. Grupp, H. J. Lezec, T. W. Ebbesen, K. M. Pellerin, and T. Thio, Applied Physics Letters 77, 1569 (2000); L. Martin-Moreno, F. J. Garcia-Vidal, H. J. Lezec, K. M. Pellerin, T. Thio, J. B. Pendry, and T. W. Ebbesen, Physical Review Letters 86, 1114 (2001). Therefore, the prevailing wisdom is that cylindrical holes do not support propagating modes when the hole diameter is smaller than ≈λ/2nh, where λ is the wavelength of incident light and nh is the refractive index of the material inside the hole. W. L. Barnes, W. A. Murray, J. Dintinger, E. Devaux, and T. W. Ebbesen, Physical Review Letters 92, 107401 (2004); W. L. Barnes, A. Dereux, and T. W. Ebbesen, Nature 424, 824 (2003). Instead, enhanced transmission is commonly associated with an excitation of surface wave resonances on the front and back surfaces of the metallic film, and an evanescent tunneling process through the holes between these resonances. T. W. Ebbesen, H. J. Lezec, H. F. Ghaemi, T. Thio, and P. A. Wolff, Nature 391, 667 (1998); L. Martin-Moreno, F. J. Garcia-Vidal, H. J. Lezec, K. M. Pellerin, T. Thio, J. B. Pendry, and T. W. Ebbesen, Physical Review Letters 86, 1114 (2001); W. L. Barnes, A. Dereux, and T. W. Ebbesen, Nature 424, 824 (2003); A. Krishnan, T. Thio, T. J. Kima, H. J. Lezec, T. W. Ebbesen, P. A. Wolff, J. Pendry, L. Martin-Moreno, and F. J. Garcia-Vidal, Optics Communications 200, 1 (2001); J. B. Pendry, L. Martin-Moreno, and F. J. Garcia-Vidal, Science 305, 847 (2004); H. J. Lezec and T. Thio, Optics Express 12, 3629 (2004).